Rewriting a browser isn't impossible either, as Firefox proves. Perhaps SO is an order of magnitude easier than a browser, but I stand by my suggestion that copying an existing application you didn't actually write is often much more difficult than expected and has led many teams to their doom.
My comparison was "writing something from scratch" vs "copying". If a team has trouble with copying, wouldn't "writing it from scratch" be even more likely to end in failure?
Also, with something like SO - you can pretty much see the models, and how they interact. That's already giving you a lot.
